This invention relates generally to the coupling of light out of a laser cavity in a controlled manner.
A laser includes two reflecting surfaces, typically one surface being more reflective than the other, an active medium between the reflecting surfaces, and a power source capable of exciting the active medium.
The power source is capable of pumping the active medium to promote a portion of its constituents to an excited state. Pumping can be electrical or optical. The active medium can be atoms or molecules in a gas, a liquid, a glass, or a crystalline solid. What is important is that the excited species be capable of emitting radiation by the process of stimulated emission; for example, that a photon passing in proximity to an excited species can stimulate that species to emit a second photon identical in energy, phase, and direction to the first.
A laser begins to oscillate when a first randomly emitted photon leaves the medium, strikes one of the reflecting surfaces and is reflected back through the medium; it then stimulates a stream of identical photons to travel along with it in an amplified beam. If these photons are reflected back into the medium, they will each be amplified. As long as the gain per round trip exceeds the losses per round trip, the result is a build-up of circulating photons. However, the population of excited state species in the medium will become depleted, due to the transfer their energy to the photon beam oscillating between the reflecting surfaces. If the pumping is continuous, the laser may settle down to a continuous operation, but if the pump is pulsed or occurs at too low a rate, the photon beam will deplete the medium and the laser will turn off again after emitting a pulse of light.
In general, a laser will begin to oscillate as soon as the gain supplied by the pump exceeds the losses imposed by scattering, transmission through the output coupling mirror, and so on, This is referred to as xe2x80x9creaching thresholdxe2x80x9d.
A useful laser output signal is usually obtained by having one of the mirrors coated so as to transmit some fraction of the light falling on it. The fraction depends upon the nature of the medium and the rate of pumping and is a critical factor in optimizing performance. If there is 0% output coupling, no output will be obtained from the laser; in this instance the light will oscillate and be very intense inside the laser cavity, but none will exit the cavity to be useful. In contrast, if there is 100% coupling, the effect is as if there are no reflectors at all; and, the laser will not operate, as it has no feedback. For any laser system, the optimum lies somewhere between these extremes of 0% and 100% output coupling.
Some media are capable of storing energy for a relatively long time; hence, their excited state species are long-lived. In this instance, it can be beneficial to block one of the reflectors to prevent photons from circulating within the cavity, while pumping the medium for an extended predetermined period of time. In this manner, the pump energy can be integrated, and when the reflector is unblocked, the intensity of the resulting laser pulse increases more than it would were the laser simple allowed to begin oscillating on its own. This process is known as Q-switching.
xe2x80x9cQxe2x80x9d refers to the cavity quality factor; a value of Q=1 implies no losses at all, and a value of Q=0 implies 100% loss. The idea is that a very low Q-cavity will allow the build-up of very large gain, and if the Q is suddenly raised, the gain will greatly exceed the loss and the laser will produce a hugely amplified output until the gain is depleted.
A common way of Q-switching is to construct a laser cavity as is shown in prior art FIG. 1. An active medium in the form of a rod 12 with Brewster angle ends is disposed between a high reflecting surface in the form of a mirror 14a, preferably being 100% reflective and an output coupling mirror having a reflectivity less than 100%. A polarizer 16 is shown between the active medium 12 and the output-coupling mirror 14b. A crystalline material in the form of a Pockels cell 18 is disposed between the polarizer 16, and a quarter-wave plate 19 is located between the Pockels cell 18 and the polarizer 16. In operation, these elements ensure that light polarized in the plane of the page of FIG. 1 can oscillate in the cavity. The oscillation of the light can be effectively blocked by having the quarter-wave plate 19 in the cavity and adjusting it so that two passes through it result in a 90 degree rotation of the plane of polarization. Thus, horizontally polarized light passing from the rod towards the output coupling mirror strikes the mirror 14b and returns towards the rod; since the light is vertically polarized it will be reflected out of the cavity by the polarizer and cannot get back to the rod to be amplified. Hence, the cavity is effectively blocked.
The Pockels cell functions as a variable wave plate activated by an applied voltage. Using a suitable voltage the net rotation on a round trip through both the wave plate 19 and the Pockels cell 18 can be made to vanish, and the laser cavity thus switches from low to high Q.
The polarizer 16, quarter-wave plate 19, and Pockels cell 18 form a Q-switch in combination with required control circuitry. The laser is operated by ensuring that the Q-Switch blocks or prevents oscillations in a controlled manner while a pulsed light source is used to pump the laser rod 12. Typically the Q-switch is maintained in a closed state where Q=0 until the pump pulse is terminated, and the rod has accumulated a significant portion of the energy from the pump. By opening the Q-switch such that Q  greater than 0, the light can circulate and due to the very high laser gain, the buildup in the circulating intensity is very fast and very large. The partially transmissive output coupling mirror 14b allows a portion of this circulating power to escape on each round trip; this constitutes a useful laser output signal. The pulse intensity and duration are only indirectly controllable, and result from a combination of how quickly the Q-switch can open, and how quickly the gain stored in the laser rod 12 is depleted by the circulating laser beam.
One drawback to the prior art system described heretofore, is that is suffers from having an output pulse profile that is determined by the opening time of the Q-switch and the depletion of the gain from the rod; this is typically a single large spike of a few tens of nanoseconds in duration. For many applications this is too much energy too quickly and has been known to cause problems in various material processing situations.
U.S. Pat. No. 4,630,275 in the name of Rapoport discloses a controlled slow Q-switch. By applying a staircase-shaped control signal to a laser Q-switch, a plurality of laser pulses are emitted with controlled energy and time separation. Compared with conventional Q-switching, the invention enables the laser to emit pulses with shorter time intervals, narrower line widths, higher output energy, and more uniform power density across the laser beam cross section. Rappaport provides a tri-level voltage input signal in order to obtain three short uniform Q-switched pulses from the output of the laser cavity.
U.S. Pat. No. 4,660,205 which discloses a technique for achieving extremely short laser pulses uses pulse-transmission mode (PTM) Q-switches. This device also uses a Pockels cell and polarizer to rapidly drain the energy from a laser cavity. In essence, the apparatus amounts to a high-speed, voltage-variable mirror, whose reflectivity can be changed rapidly between 0 percent and 100 percent (see Solid-State Laser Engineering, W. Koechner, Springer-Verlag, N.Y. (1976), pp. 441 ff). A similar technique is used for cavity dumping of CW-pumped solid-state lasers (op. cit., pp. 444ff). Notwithstanding, the variable voltage mirror disclosed in the ""205 patent does not provide the useful functionality of a voltage variable output-coupling mirror.
Another form of laser that a particular embodiment of the instant invention relates to is a novel cavity dumped laser which provides for the acquisition of controllable pulses. FIG. 3 illustrates a conventional prior art arrangement of such a laser known as a xe2x80x9cdouble-pass cavity dumperxe2x80x9d. In this system, the laser cavity is constructed using 100% reflective mirrors, arranged in a folded cavity configuration. At the centre of the folded region is a Bragg cell 30 which is comprised of a small block of light transmissive material with an acoustic transducer affixed to one side. A high power RF waveform of several watts is applied to the transducer, which in turn produces an acoustic wave characteristic of the waveform, in the Bragg cell 30. The cell is positioned so that the laser beam passes through it and the acoustic wave travels at right angles to the direction of the laser beam. For optimum efficiency the laser beam is polarized in a direction perpendicular to the direction of travel of the acoustic beam. The laser beam is diffracted by the acoustic beam and splits into two sub-beams, one continuing to propagate along its original direction and the other deflected at an angle dependent upon the acoustic frequency of the acoustic wave within the Bragg cell 30 along a different path. The relative intensity of the two beams can be modified and controlled by varying or maintaining the intensity of the RF signal used to generate the acoustic wave. The useful laser output signal is obtained by utilizing the deflected portion of the beam. Such a system is used in lasers where the active medium is not long-lived. Once consequence of this is that the medium is not an effective means of storing energy, and efforts to generate high intensity laser pluses rely on storing the energy as circulating photons instead. Thus when circulating power has reached its peak value, the energy is dumped as completely as possible. Since the round trip time of a laser cavity is typically 10 nanoseconds, even a 99% efficient cavity will loose over 90% of its energy in a few microseconds; thus it is evident that this method is only effective for generating very short laser pulses. Since the laser cavity is always oscillating, it does not obtain the very large output pulses obtained with a Q-switch, and the photon losses between pulses severely limits the overall efficiency of most systems.
In both the conventional Q-switch and cavity dump laser systems, it is desired to switch in a nearly binary manner, to provide pulses generated by allowing substantially all of the energy in the lasing medium to be used in an on-off, all or none manner, substantially instantaneously. Heretofore, various switching schemes have been developed to provide quick transitions between switched states.
However, in accordance with this invention, control of the amount and duration of the energy provided in a laser output pulse is afforded by effectively providing a variable mirrored cavity, wherein the ratio of light that is extracted from the cavity to the remaining portion within the cavity can be precisely controlled. It should be understood that although the mirror""s reflectivity in the instant invention is fixed, the effect of controlling the amount of light that is extracted from the cavity, precisely, by for example controlling the polarization state within any increment, is tantamount to providing a variable reflectance output mirror. Heretofore, one, two or three voltage levels via switches controlled a variable wave plate to effect Q-switching; however, now, by practicing the teachings of the instant invention, a Pockels cell, for example can be controlled by a computer generated waveform to provide one or more xe2x80x9cdesignerxe2x80x9d output pulses that have a desired shape and duration, such that the energy within the pulse is dispersed according to the computer generated input waveform driving the system. In prior art optical cavities, varying the voltage applied to the Pockels cell is only a means of varying the cavity loss; the output coupling is fixed in conventional systems. Therefore any xe2x80x9cpartialxe2x80x9d or xe2x80x9csteppedxe2x80x9d switching is essentially wasteful, since while useful output is extracted from the output mirror, a similar if not larger beam will bounce off the polarizer as waste. In the laser system of the instant invention the energy remains stored in the long-lived medium unless extracted as useful output.
Conveniently, the device and method of the invention provides a means of controlling the energy distribution within an output pulse, as well as providing a means of controlling the duration of a pulsed signal. Yet still further, the invention allows a voltage input waveform to be modified in real-time while cutting, or other procedures are occurring, in order to optimize a particular task or to vary a particular task in real-time.
Yet still further, this system conveniently provides for real-time genetic learning algorithms to be utilized such that optimization is afforded while the device is in a learning mode of operation. Feedback means can be provided such that the laser""s output pulse can be varied in accordance with its result, for example in cutting or ablating applications.
In contrast the aforementioned Q-switched devices are far more limited in producing one desired output response by applying one, two or three switched voltage levels as inputs to produce one, two or three output pulses respectively, and are inherently wasteful.
Heretofore, it has been an object of Q-switched lasers to provide a device that performs like a high-speed, voltage-variable deflecting mirror or shutter, whose transmission can be changed rapidly between 0 percent and 100 percent. In contrast to this, and in accordance with the teachings of this invention, it is an object to provide a device that performs as if it had voltage variable output coupling mirror, variable in such a manner as it could change its reflectivity, through a plurality of pre-programmed transitions between 0 and 100 percent in such a manner as to characterize a complex input computer generated waveform having a plurality of transitions. In preferred embodiments of the invention, the complex waveform ensures that switching does not occur too quickly so that the laser pulse is not Q-switched but has a predetermined shape and duration. In contrast with the Q-switched laser operation, it is not desirous to have the laser medium dump all of its stored energy quickly as possible, but rather distributed in a controlled manner for the duration of the output pulse. For example, if it is desirable to obtain an output pulse with a gradually rising intensity, it is necessary to closely control the early portion of the complex waveform to ensure that only a small amount of energy is extracted from the medium during the early part of the pulse, in order that a larger amount is available for the later, high intensity, portion of the pulse.
It is an object of this invention, to provide an optical device that is capable of offering control of the intensity, duration, and pulse envelope from a laser, over durations of a few tens of nanoseconds to hundreds of milliseconds, wherein the energy distribution of an output pulse is dependent upon a programmed input waveform stored within a memory device.
In accordance with the invention, a laser cavity is provided, comprising: two substantially totally reflecting surfaces for preventing light incident thereon from exiting said surfaces; a long-lived lasing medium having a lifetime of at least two orders of magnitude longer than the cavity lifetime disposed within an optical path between the two reflecting surfaces for emitting light energy in a direction to be reflected by said reflecting ends;
an energy source for providing energy to the lasing medium;
a polarizer within the cavity for permitting passage of light energy having a first direction of polarization and for reflecting light of an orthogonal polarization to said first direction;
a quarter wave plate disposed within the cavity between the polarizer and one of the two reflecting surfaces for rotating the polarization of light passing therethrough a variable wave plate for variably rotating the polarization of light passing therethrough in a controlled manner and being controllably adjustable so as to provide a variable degree of elliptical polarization to be induced in a controlled manner so that the amount of light deflected by the polarizer is controllably variable;
a waveform generator for generating an amplified complex signal having a plurality of maxima and minima corresponding to a single pulse laser output signal and for providing an amplified complex signal to the variable waveplate to yield the single pulse laser output signal.
In accordance with the invention there is further provided, a laser cavity comprising: two reflecting mirrors which form ends of the laser cavity, the mirrors both being substantially non-transmissive to light propagating within the cavity and for reflecting all light incident thereon within the cavity;
an active long-lived lasing medium having a lifetime of at least two orders of magnitude longer than the cavity lifetime disposed along a path within the cavity between the two mirrors;
a polarizing steering means for passing light in a first predetermined polarization state, incident thereon therethrough along a path towards one of the mirrors, and for deflecting light in an other polarization state to another path away from said one of the mirrors to an output port of the laser cavity disposed between the two mirrors;
means for providing a computer generated complex amplified signal for generating a single laser pulse;
and, means responsive to the complex amplified signal for variably controlling the state of polarization between the active lasing medium and one of the mirrors to effect light to be variably directed to one of the output of the laser cavity and the path within the cavity between the two mirrors and for controlling the pulse shape of the output pulse of light corresponding to the computer generated complex amplified signal.
In accordance with another aspect of the invention, a method is provided for producing a controllable laser signal comprising the steps of:
providing a cavity with two mirrors;
providing a long-lived lasing medium having a lifetime of at least two orders of magnitude longer than the cavity lifetime for emitting light energy within said cavity;
providing energy to the lasing medium;
providing a polarization control circuit for extracting light from within the cavity in a controllable, non-binary manner in dependence upon its polarization state;
controlling the polarization of light within said cavity in a variable, non-binary, manner by providing a computer generated complex waveform signal to an amplifier, said complex waveform signal having plurality of maxima and a plurality of minima;
amplifying said complex waveform to yield a complex amplified signal being an amplified representation of the complex waveform; and
providing said complex amplified signal to control the polarization control circuit.
In accordance with another aspect of the invention, a method of controlling the shape of a pulse of light extracted from a laser cavity having a long-lived lasing medium having a lifetime of at least two orders of magnitude longer than the cavity lifetime is provided comprising the steps of:
providing a computer generated waveform for characterizing the pulse shape; characterizing the computer generated waveform in the form of an amplified signal;
applying the amplified signal to a controller within the laser cavity for controlling the pulse shape and duration of light extracted from a laser cavity.
There is further provided, a laser cavity comprising two reflective surfaces;
a lasing medium for emitting light energy within said cavity;
a waveform generator responsive to a computer generated waveform for providing a waveform corresponding to a desired output response;
an amplifier for amplifying the waveform and for providing an amplified waveform;
a control circuit for extracting light from within the cavity in a controllable, non-binary manner in dependence upon the computer generated waveform.